Abstract

Underthrusting of the Indian lithosphere beneath the Himalayas occurs during the Quaternary period along a gently north-dipping main basal detachment (main Himalayan thrust: MHT), from which the southernmost emergent ramp (main frontal thrust: MFT) branches. Historical seismicity shows that slip on the MHT is frequently accommodated through M > 8 shallow earthquakes, but shows a seismic gap in western Nepal. This absence of major historical earthquakes in western Nepal can be explained either by an aseismic slip on the MHT or a long-lived elastic strain accumulation. To test these hypotheses, the present-day displacement field has been measured for a GPS network formed of 35 sites. The updated solution presented in this paper combines data from 1995, 1997, 1998 and 2000 measurements. The lack of deformation (less than 3 • 10^(−8) yr^(−1)) through the outer belt does not fit with a regional aseismic slip along the southern part of MHT. A less than 3 mm yr^(−1) aseismic slip could nonetheless affect restricted areas of the outer belt. In contrast, a strain accumulation of more than 30 • 10^(−8) yr^(−1) is measured south of the Higher Himalayas, in a zone where an intense microseismicity reflects a stress build-up. It is presumably generated by locking of the aseismic creep that occurs along the MHT beneath the Higher Himalayas and Tibet. The displacement field is simulated by a dual-dislocation model that takes into account the pattern of microseismicity, and particularly a segmentation between central and western Nepal. The best fit between the measured and simulated displacement fields is obtained with 19 mm yr^(−1) thrust and 0–1 mm yr^(−1) dextral strike-slip components along a 117°NE dislocation locked to a depth of 20–21 km beneath western Nepal, and 19–20 mm yr^(−1) thrust and 0–2 mm yr^(−1) dextral strike-slip components along a 108°NE dislocation locked to a depth of 17–21 km beneath central Nepal. The width of the locked zone between the main frontal thrust and the creeping zone is of the same order, but rather greater, in western Nepal than in central Nepal. Therefore it is expected that M > 8 earthquakes could occur in western Nepal.